canon rumors FORUM

I don't use any ultra-wide lenses but this may be a good article which compares Zeiss 15 f2.8 with one of the best Canon's lens – 17mm f4 TS. http://www.fredmiranda.com/forum/topic/1207619. I think Canon's TS has superb built quality and corner-to-corner sharpness, and of course tilt-shift capabilities. Zeiss probably isn't as sharp at corners but some people note a special Zeiss microcontrast.

According to Ken Rockwell, Zeiss 15 f2.8 is much better as compared to other ultrawide Canon lenses, such as EF 14mm f2.8. Lloyd Chambers says Zeiss 15mm f2.8 has exceptional contrast, exceptionally low distortions, high resistance to glare, absence of chromatic aberrations, and in general delivers a better image quality than Nikon 14-24.

Overall, Zeiss 15mm f2.8 seems to be an amazing performer. Is there a particular Canon lens you're comparing it with?

From an edge to edge sharpness, the Zeiss 15mm wins hands down, compared to Canon's equivalents (excluding the TS range). The TS lenses from Canon are probably on par with the Zeiss. But, they are designed with a much larger image circle, to account for the shift that the lens can do (think of a FF lens on a small sensor).

For my testing (and I realise it's not perfect) I simply hired a few lenses and took some shots of the stars, which quickly sorted out which had coma or not (coma creates edge softness in a lens, not really correctable in PS or Lightroom). For example, the Canon 24mm f1.4 is a beaut lens, in the center, but soft around the edges. Point it towards the stars and you will see why

That said, it'll depend on what the lens is intended for. Are you wanting wide open usage, or can to stop the lens down a bit?